Normally, visual access to a closure member of a valve is blocked by the valve housing and by the process line or equipment in which the valve is installed. Further, in many instances, the valve is inconveniently located, and there is a need to ascertain the position of the valve closure member at a location removed from the valve. For these reasons, many prior art devices have been developed for not only detecting the position of a valve closure member, but also for electrically communicating such information to a remote location.
Perhaps the most common contemporary commercial practice is to use detectors which sense the end limits of the closure member movement and which open or close an electrical switch in response thereto. Typically, the closure member is moved by an actuating shaft which extends through an actuator housing. In one prior art arrangement, this extesion of the actuating shaft out of the actuator housing is used to mechanically interconnect the closure member with a pair of rocker arms actuated limit switches. The actuating shaft moves the rocker arms at the end limits of closure member movement; and the movement of the rocker arms is used to open or close electrical contacts. The contacts, in turn, open or close an electrical circuit to produce an output signal indicative of whether the closure member is in an open or closed position.
While used quite extensively, mechanically actuated limit switches, such as the rocker arm type switches discussed above, suffer from several shortcomings which limit their effectiveness in many industrial environments. First of all, these prior art switches are often insufficiently sealed (if sealed at all). As a consequence, the switch contacts occasionally malfunction due to exposure to dirt and/or process materials. Additionally, the rocker arms, or other mechanical components, of these prior art switches are many times directly exposed to process material spills. Many such process materials are corrosive and will corrode the mechanical components or inhibit their mechanical movement. For example, viscous fluids, such as corn syrup, may spill and "gum up" rocker arms to such an extent as to prohibit switch operation.
Mechanically actuated limit switches are also susceptible to mechanical abuse. It is not uncommon, as an example, for workers to climb upon or step on limit switches as they attempt to access other process equipment components. The weight of a typical worker may well bend the rocker arms of the above described limit switches and render such switches inoperative.
The size of mechanical limit switches also poses limitations upon their use. Mechanical limit switches generally extend above valve actuators and have relatively high profiles. On occasion, such as when other process equipment or low ceiling dimensions must be taken into consideration, these high profiles restrict placement of the actuator.
It is also known in the art to detect the position of a valve closure member with magnets and one or more magnetic reed switches. One of either the magnets or the reed switches is positioned to move with the closure member, with the other positioned on the valve body. For example, in U.S. Pat. No. 3,538,948 to Nelson et al, a gate valve is disclosed wherein permanent magnets are affixed to the movable gate member. A non-movable sleeve member containing a pair of upper and lower spaced magnetic reed switches is disposed within the gate member. As the gate member is moved between open and closed positions, the permanent magnets affixed thereto are transported in proximity to the reed switches so as to cause the reed switches to close and to complete an electrical circuit. The magnets and reed switches are positioned to bring the magnets in close proximity with the lower reed switch when the gate member is in the closed position and to bring the magnets in proximity with the upper reed switch when the gate member is in the open position.
A similar arrangement wherein magnets and magnetic reed switches are used to detect the position of a gate valve member is disclosed in U.S. Pat. No. 3,789,875 to McGee. In McGee, upper and lower reed switches attached to the valve body are activated by magnets embedded in a tubular actuating stem for the gate member.
In U.S. Pat. No. 4,093,000 to Poff, magnets are supported within a tubular shell that raises, lowers and rotates with the valve stem of a rising stem valve. This movement of the tubular shell positions the magnets in proximity with reed switches affixed to a sleeve disposed with the shell to selectively activate the reed switches and to produce electrical signals indicative of the position of the valve closure member.
A removable position detection device for a valve is disclosed in U.S. Pat. No. 3,522,596 to Fowler et al. The Fowlder et al device includes a cylindrical sleeve portion connecting upper and lower circular plates adapted for attachment to the side of a valve body. Reciprocating movement of the valve actuating stem is used to rotate a cantilevered arm supporting a magnet on its free end. The rotational movement of the cantilevered arm moves the magnet over magnetic reed switches, which reed switches are fixedly mounted on a terminal board. In an arrangement for detecting the angular position of a rotating shaft, Fowler et al discloses a magnet secured to the shaft. The magnet is rotated past a plurality of spaced magnetic reed switches fixedly secured to a stationary plate.
The magnetic reed type limit switches described above has been subject to many of the same shortcomings previusly discussed in connection with mechanically activated limit switches. The reed switches themselves are generally sealed hermetically. However, the connections between the reed switches and the attached conductors are exposed to dirt and other contaminants within the industrial environment. As previously noted, many of the potential contaminants are corrosive and exposure of these connections thereto may result in a malfunction.
Further, when the prior art valves containing reed switches are subjected to vibration over extended periods of time, the securement between the reed switches and the valves (or mounting plates) is subject to failure. Even slight relative movement between the valve and the reed switch may cause a system malfunction.
Additionally, precisely securing and positioning reed switches to valve bodies or mounting plates is a time consuming activity requiring tedious and precise manual labor. Hence, such reed switch securement adds considerable cost to the valve. Also, once the reed switches are secured, their position is not readily adjustable to vary the trip point at which the switch is actuated. Moreover, the valves using such switches are not completely interchangeable with similar valves without the switches; and retrofitting of existing valves is difficult.